1. Field of the Invention
The present invention relates to a method for blackening a Ni-Fe shadow mask and a mesh belt type blackening lehr.
2. Description of the Related Arts
Blackening processing, namely, processing for forming an oxide film on the surface of a shadow mask of a cathode ray tube for use in a color television is performed to improve a heat diffusion and prevent the shadow mask from being rusted.
Conventionally, blackening processing is carried out in an atmosphere of humidification exothermic type gas or an atmosphere of mixture gas of air and humidification exothermic type gas.
In the conventional blackening processing, if the shadow mask is made of aluminum killed steel, an oxide film having a desired radiation coefficient can be formed, but the aluminum killed steel has a great coefficient of thermal expansion and thus, is unsuitable as a material of the shadow mask of a cathode ray tube for use in a color television because the shadow mask has a great radius of curvature. Instead of the aluminum killed steel, Invar material of Ni-Fe is used as a material of a shadow mask because it has a small coefficient of thermal expansion. Since it is difficult to oxidize the Invar material because it contains Ni, so that the blackening processing does not provide a sufficient radiation coefficient.
In blackening a shadow mask of 36% Ni-Fe at a temperature of 600.degree. C. for 20 to 30 minutes, a light brown or purple brown oxide film having a radiation coefficient of 0.32 to 0.38 is obtained in an atmosphere of humidification exothermic type gas and a blue ash oxide film having a radiation coefficient of 0.45 to 0.6 is obtained in a mixture gas of air and humidification exothermic type gas. A film of Fe.sub.2 O.sub.3 is formed on the surface layer of an oxide film in both cases.